1. Field of the Invention
The present invention relates to friction materials suitable for use in such applications as brake pads and brake linings for automobiles and various types of industrial machinery.
2. Prior Art
Friction materials such as disk brake pads and brake linings, particularly those used in vehicles such as automobiles and motorcycles, are required to have not only a sufficiently high coefficient of friction (good effectiveness), but also various other properties. For the intentions of the driver to be accurately manifested as a braking force on the vehicle, a quality referred to hereinafter as “excellent brake feel,” it is essential that the friction material undergo little elastic deformation when subjected to stress and that the coefficient of friction have a linear relationship with the stress (i.e., that the strain incurred be linear). Further requirements include low fluctuation in the friction coefficient (a linear torque) even when the opposing material such as a disc rotor or brake drum is turning at high speed or is at a high temperature, little tendency to damage the disc rotor (low disc rotor attack), resistance of the friction material to wear, and little tendency for metal components from the abraded rotor to aggregate on the surface of the friction material and cause abnormal wear of the disc rotor (low metal pickup). The desire has been to achieve these and other required properties in a good balance.
Of these various required properties, to ensure in particular a high friction coefficient under high loading, JP-A 8-135703 discloses a friction material for stainless steel rotors which is composed of a fibrous base, a resin binder and a filler, and which includes as metal components 30 to 60 wt % of at least one type of copper-based fiber selected from among copper fibers and brass fibers and 3 to 10 wt % of stainless steel fibers or powder, and additionally includes 2 to 10 wt % of graphite. All of these amounts are based on the overall friction material.
Also, particularly to obtain a friction material having a linear torque (that is, to prevent a decrease in the coefficient of friction under the application of a high hydraulic pressure), JP-A 1-320329 discloses a friction material composition which is prepared by blending reinforcing fibrous materials, specifically 5 to 15% of steel fibers or stainless steel fibers, and also ceramic fibers and organic resin fibers, with organic fillers, inorganic fillers, metal powders and friction modifiers, and binding these components with a thermoset resin binder, and which additionally contains 0.1 to 2.0 vol % of an alloy powder having a microvickers hardness of at least 1,000.
However, friction materials endowed with all of the various above properties required of friction materials for disc brake pads, particularly those used in vehicles such as automobiles, have not hitherto been developed. Hence, there is a need for friction materials which achieve a good balance of these required properties.